In positive displacement compressors employing suction and discharge valves there are both similarities and differences between the two types of valves. Normally the valves would be of the same general type. Each valve would be normally closed and would open due to a pressure differential across the valve in the direction of opening. The valve may be of a spring material and provide its own seating bias or separate springs may be employed. Since the suction valve(s) open into the compression chamber/cylinder they generally do not have valve backers in order to minimize the clearance volume and thus deflection of the valve is not physically limited. Discharge valves normally have some sort of valve backer so as to avoid excess movement/flexure of the discharge valve. Ignoring the effects of leakage, etc., an equal mass of gas is drawn into the compression chamber and discharged therefrom. However, the suction stroke takes place over, nominally, a half cycle whereas the combined compression and discharge stroke makes up, nominally, a half cycle. In the case of the suction stroke, the suction valve opens as soon as the pressure differential across the suction valve can cause it to unseat. Typically, the pressure differential required to open the suction valve is on the order of 15-35% of the nominal suction pressure. In the case of the compression stroke, compression continues with the attendant reduction in volume/increase in density of the gas being compressed until the pressure of the compressed gas is sufficient to overcome the combined system pressure acting on the discharge valve together with spring bias of the valve member and/or separate springs. Typically, the pressure differential required to open the discharge valve is on the order of 20-40% of the nominal discharge pressure. Accordingly, the mass flow rate is much greater during the discharge stroke.
By design, suction valves have a much lower seating bias than discharge valves. The low seating bias is essential due to the fact that valve actuation is initiated by the force resulting from the pressure differential across the valve. In the case of suction valves, opening generally occurs at pressures that are much lower than in the case of discharge valves. Therefore, only small pressure differences, and hence small opening forces, can be created for suction valves relative to potential pressure differences and opening forces for discharge valves. Even a small increase in the pressure differential across the suction valve results in a large percentage increase in the pressure differential across the valve. In contrast, an equal increase in the pressure differential across the discharge valve results in a much smaller percentage increase in the pressure differential because of the substantially higher nominal operating pressure.
The opening force, F, on a valve is given by the equation EQU F=P.cndot.A
where P is the pressure differential across the valve and A is the valve area upon which P acts. It should be noted that the direction in which the pressure differential acts changes during a complete cycle so that, during a portion of a cycle, the pressure differential provides a valve seating bias. When A is held constant, it is clear that a change in F is proportional to a change in P, or, more specifically, the percentage change in F is proportional to the percentage change in P. For example, assuming an operating condition where suction pressure is 20 psia and discharge pressure is 300 psia, at a typical overpressure value of 35%, the cylinder will rise to 405 psia before the discharge valve opens. In contrast, at a typical underpressure value of 30%, the cylinder pressure will drop to 14 psia, before the suction valve opens. If the pressure differential required to open both valves is increased by 10 psia, the discharge overpressure value increases to 38% from 35% while the suction underpressure value increases to 80% from 30%. Thus, we can expect the opening force on the suction valve to increase 167%.
Particularly because of the effects of the clearance volume, the change in pressure differential across the suction valve would not increase very rapidly since the cylinder is initially charged due to the compressed gas from the clearance volume and is then acting as a vacuum pump until the suction valve opens. Specifically, the inflow of gas to the cylinder is typically designed to occur during the last 95% of the combined expansion and suction stroke. In contrast, the compression chamber pressure rises rapidly as the compression stroke is being completed and the pressure can continue to rise during the discharge stroke if the volume flow exiting the cylinder does not match the rate of reduction in the compression chamber volume. Typically, the outflow of gas from the cylinder occurs during the last 40% of the combined compression and discharge stroke. Any substantial change in one or more of these relationships can result in operational problems relative to the valves.
Another complicating factor arises from the fact that under typical operating conditions, lubricating fluid (oil) coats all internal surfaces of a compressor, including the suction and discharge valves and valve seats. The associated problems as to improving discharge efficiency as related to the discharge valve have been addressed in U.S. Pat. No. 4,580,604. In the case of a discharge valve, the cylinder pressure must overcome the system pressure acting on the discharge valve, the spring bias on the valve and any adhesion of the valve to the seat. Accordingly, the adhesion of the discharge valve to the seat represents an over pressure and therefore an efficiency loss.
A typical reciprocating compressor will have a valve plate with an integral suction port and suction valve seat. When in the closed position, the film of oil present between the suction valve and its seat is very thin, on the order of a few molecular diameters. This is in part due to the fact that compression chamber pressure acts on and provides a seating bias for the suction valve during the combined compression and discharge stroke. In normal operation, the opening force applied to the suction valve is provided by a pressure differential across the valve that is created as the piston moves away from the valve during the suction stroke. Typically, the opening force needs to be large enough to overcome the resistance to opening caused by valve mass (inertia) and any spring or other biasing forces. The force also needs to be substantial enough to dilate and shear the oil film trapped between the valve and seat. Factors that influence the force necessary to dilate and shear the lubricant film include: the viscosity of the lubricant film, the thickness of the oil film, the inter-molecular attractive forces between the lubricant molecules, the quantity of refrigerant entrained in the oil film, the materials of construction of the suction valve and/or valve seat, and the rate of refrigerant outgassing.
In traditional refrigerant-compressor applications using mineral-based (MO) or alkylbenzene (AB) lubricants, the resistance to opening caused by the lubricants is negligible as indicated by the relatively small pressure differential that is required to initiate valve opening. This is due, in large part, to the fact that MO and AB lubricants exhibit relatively low viscosity, low inter-molecular forces and good solubility with refrigerants over the entire range of operating conditions.
Newer, ozone-friendly refrigerant-compressor applications utilize polyol ester (POE) lubricants. When compared to MO or AB lubricants, POE lubricants can exhibit extremely high lubricant viscosity and poor solubility with HFC refrigerants such as R134a, R404A, and R507, particularly under low operating pressures and/or temperatures. The relatively high viscosity of POE's can cause a substantial increase in the force necessary to dilate and shear the oil film trapped between the valve and seat. Additionally, POE lubricants are very polar materials and hence have a strong molecular attraction to the polar, iron-based materials that are typically used to manufacture valves and valve seats. The mutual attraction of the materials of construction and the POE further increases the force necessary to separate the valve from the valve seat.
In order to generate the increase in force needed to separate the suction valve from its valve seat, the pressure differential across the valve must be increased with an accompanying delay in the valve opening time. When the suction valve does finally open, it does so at a very high velocity. Further, aggravating this condition is the increase in the volume flow rate of the suction gas entering the cylinder resulting from the delay in the suction valve opening. The increase in the volume flow rate of the suction gas causes an increase in suction gas velocity which, in turn, increases the opening force applied to the suction valve and, hence, the velocity at which the valve opens. The increased suction valve opening velocity resulting from the combined effects of a higher pressure differential on the valve due to the delayed opening and the higher volumetric flow rate of the flow impinging upon the suction valve causes the suction valve to deflect further than intended into the cylinder bore. Without the benefit of a valve backer, as would be present in a discharge valve, valve operating stress must increase as a result of the increase in valve deflection. If the operating stress exceeds the apparent fatigue strength of the valve, then valve failure will occur.